Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Unbilled revenues	$ 71.3		$ 71.3		$ 81.1
Franchise fees and gross receipts taxes	32.6	$ 31.7	89.2	$ 88.3
Tampa Electric Company [Member]
Summary Of Significant Accounting Policies [Line Items]
Unbilled revenues	63.7		63.7		$ 53.7
Franchise fees and gross receipts taxes	$ 32.6	$ 31.7	$ 89.2	$ 88.3